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Science

Atomic structure

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An atom is made from a nucleus surrounded by electrons. The nucleus contains protons and neutrons. Isotopes are atoms that have the same number of protons, but different numbers of neutrons.

An early model of the atom was the plum pudding model. It was disproved by Rutherford's scattering experiment and replaced by the nuclear model.

Structure of the atom

You should be able to describe the nuclear modelnuclear model: The scientific idea that an atom has electrons surrounding a nucleus that contains protons and neutrons. of the atom [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ].

The nuclear model

atoms have a small central nucleus surrounded by electrons

The structure of the atom

Atoms contain three sub-atomic particles called protonsproton: A proton is a small particle with a positive charge found in the nucleus of the atom., neutrons and electrons [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ]. The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in energy levels around the nucleus.

The table shows the properties of these three sub-atomic particles.

Properties of sub-atomic particles

ParticleRelative massRelative charge
Proton1+1
Neutron10
ElectronAlmost zero–1

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:

  • If an atom loses one or more electrons, it becomes a positively charged ion
  • If an atom gains one or more electrons, it becomes a negatively charged ion

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